In recent years, there has been a reduction in population of workers in an aging society with a falling birthrate. Because of such a reduction in population of workers, there has been demand for a robot arm which (i) can manufacture a product in cooperation with a human at a production site or (ii) can support a daily life of an old person or a daily life of a handicapped person.
A robot arm constituted by rotational joints has an arrangement in which a plurality of rotational joints 103 and a plurality of arm sections 102 are, alternately, connected to each other in series between a base 100 and an end effector 101 (see (a) and (b) of FIG. 19). With the arrangement, in a case where a position and orientation of the end effector 101 are determined, rotational angles of the plurality of rotational joints 103 are generally determined uniquely. That is, there is a problem that it is not possible to determine arbitrarily the position of the end effector 101 and a position of each of the rotational joints 103. For this reason, in a case where the end effector 101 is moved from a position illustrated in (a) of FIG. 19 to a position illustrated in (b) of FIG. 19, for example, the plurality of arm sections 102 become close to each other. In this case, there is an increase in risk that an object located in the vicinity of the plurality of arm sections 102 might be sandwiched between the plurality of arm sections 102. Further, since one of the plurality of rotational joints 103 is moved upward in FIG. 19 from a straight line (shown in a dashed line) connecting the base 100 and the end effector 101 to each other, there is an increase in risk that the one of the plurality of rotational joints 103 becomes in contact with, or crashes against an object located in the vicinity of the robot arm. Such an arrangement is not suitably applicable to a robot arm which operates in the vicinity of a person in a daily life or the like.
In order to reduce such risks, there has been proposed a robot arm having a direct acting extensible and retractable arm mechanism which employs a direct acting joint in place of a rotational joint. The direct acting extensible and retractable arm mechanism employs linear extensible and retractable motion, and has been applied to a crane vehicle, a ladder truck, etc. In order to ensure a high rigidity, the linear extensible and retractable arm mechanism, applied to the crane vehicle, the ladder truck, etc., is such that a mechanism of an arm section becomes larger with increasing distance from a hand section toward a root section serving as a base section. Accordingly, it is necessary for the direct acting extensible and retractable arm mechanism, applied to the crane vehicle, the ladder truck, etc., to have the base section and the arm section, both of which are large and heavy.
Further, Patent Literatures 1 through 3 also describe a robot arm having a linear extensible and retractable arm mechanism. However, such a robot arm has danger in a case where the robot arm operates in the vicinity of a human in a daily life or the like. Further, a general direct acting mechanism used in an industrial machine keeps its linear mechanism all the time. For this reason, in a case where the direct acting mechanism is stored, the direct acting mechanism thus stored protrudes from a main body of the robot arm. That is, there remains danger for a human in a case where the direct acting mechanism operates in the vicinity of the human in a daily life or the like.
Patent Literature 4 describes an arm employing a direct extensible and retractable arm mechanism, which arm has a low risk that the arm might be in contact with or crush against an object located in the vicinity of the arm. However, this arm ensures its rigidity only in a direction of gravitational forth. That is, the arm is not suitably used as a robot arm that is used in a daily life or the like, in which (i) realization of an arbitrary position of the arm and an arbitrary posture of the arm are required, and (ii) various operations are required, such as holding an object with an end effector, pressing a button, and exhibiting force not only in the direction of gravitational force but also in directions of 6 axes of a position and a posture. Furthermore, since an overload concentrates on an axis, it is difficult to ensure a high rigidity of the arm. Moreover, since a lot of gaps are formed on an upper surface between links, it is likely that (i) a finger is sandwiched inside the direct acting extensible and retractable arm mechanism and (ii) dust enters inside the direct acting extensible and retractable arm mechanism. That is, this arm cannot be suitably used as a robot ram used in the vicinity of a human in various environments, e.g., when being used in a daily life.